1. Field of the Invention
This invention relates generally to apparatus for metering and/or feeding fluid material, particularly colorants and plasticizers for injection molded/extruded polymers. More particularly, this invention relates to compressible tube type pumps or peristaltic metering pumps as they are more commonly known and improvements in extending tube life.
2. Description of Related Art
Conventionally, rotary peristaltic pumps have heretofore utilized curvilinear metering tubes as described in U.S. Pat. No. 2,123,781. Also, described in U.S. Pat. Nos. 3,101,674 and 3,582,234 are liquid metering tubes which are in part linear, but which have bends or curves in the metering tube within the zone which is compressed in order to transport the metered liquid. The non-linearity or the bending of the metering tube increases the stress on the tube resulting in premature failure. Premature failure of the metering tube is of concern due to the potential loss of material and the possible damage to equipment. This is especially important in the polymer colorant and plasticizer area due the high cost of the material.
Stress on the metering tube is believed to be concentrated at the sharpest point of the bend of the metering tube over which the compression rollers pass. This is believed to be due to the stretching of that section of the tube which is toward the outside of the bend and the compression of that section of the tube which is toward the inside of the bend during compression of the tubing. The repeated stretching of the outside of the metering tube which is under tension is believed to cause the failure of the metering tube at the location of the bend.
U.S. Pat. No. 4,529,106, discloses a metering and feeding unit for fluid materials, such as Gunite, a sand cement mixture. A vertically oriented elastic-walled tube is collapsed by tube means for engaging a portion of the tube wall to collapse the tube against itself. The pumping action of the rollers travelling over the length of the tube assists the normal flow of material due to gravity. A valve is provided to allow entry of the fluid material into the feed end of the tube at periodic intervals. The valve works in coordination with a power-driven assembly so that when the tube is initially engaged to collapse it, that portion of the tube is substantially free of material.